Monitoring system for stereophonic sound channels



J. F. BYRD 2,791,629

MONITORING SYSTEM FOR STEREOPHONIC SOUND CHANNELS Filed Sept. 30, 1953 yMay 7, 1957 United Smtes Patent O i;

MGNITORING SYSTEM FR STEREOPHONIC SOUND CHANNELS John F. Byrd, Ashland, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 30, 1953, Serial No. 383,351

6 Claims. (Cl. 179-1) This invention relates to sound reproduction in general, and particularly to a system for monitoring stereophonic theater sound channels.

With the advent of stereoscopic motion pictures which have been accompanied by stereophonic multiple channel sound systems in theaters2 it has been found desirable to monitor each sound channel separately as well as certain groups or combinations of channels. In the present types of multiple channel sound systems, the normal monitoring circuits create cross-talk between channels or change the sound level either in the main theater speakers or in the monitoring speaker or speakers.

The present invention is directed to a system which is capable of monitoring each sound channel separately to determine if each channel is functioning properly, and to monitor one or more channels in combination for checking the general sound quality during the exhibition of the pictures. Since the complete sound spectrum is composed of several sound channels divided between as many stage speakers, this monitoring system makes use of a two-section high impedance T pad bridged across each sound channel. Furthermore, the terminating resistor for each pad is the input loading resistor on the amplifier. Thus, there is high isolation between channels which eliminates cross-talk and reverse bridging effect to lthe input of the monitor amplifier, which gives approximately the same level in the monitor speaker regardless of the number of channels being monitored.

The principal object of the invention, therefore, is to facilitate the monitoring of a stereophonic multi-channel theater sound system.

Another object of the invention is to provide an im proved monitoring system for a stereophonic multi-channel theater sound system.

A further object of the invention is to provide an improved monitoring system for a plural channel sound system which does not create cross-talk between channels nor change the sound level in any of the speakers.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization and the mode of its operation will be better understood by referring to the following description, read in conjunction with the accompanying drawings, forming a part hereof, in which:

The single gure is a diagrammatic drawing of a multichannel theater sound system embodying the invention.

Referring now lto the drawing, five signal sources are indicated. Sources Nos. l, 2, and 3 are magnetic sound reproducers, while sources Nos. 4 and 5 are photographic sound reproducers, although it is to be understood that either a magnetic or a photographic reproducer may be used at any position as far as the present invention is concerned.

The No. l magnetic reproducer is connected through conductors to a speaker 11 positioned on the left-hand side of a motion picture screen. Bridged across conductors 10 is a T pad 12 composed of resistors R-1, R-2,

y 2,794,629 'PatentedV May 7, 1957 and R-3. The T pad 12 is connected through a switch 13 across a matching resistor R-16 in the input of a monitoring amplifier 15 having an input transformer 16, the first tube of the amplifier being indicated at 17. The output of the amplifier 15 feeds a monitor speaker 20.

The No. 2 magnetic reproducer is connected'through conductors 21 and a double pole, double throw switch 22 to a speaker 23 centrally positioned behind the motion picture screen when the switch 22 is in its upper position. Bridged across conductors 21 is a T pad 25, composed of resistors R-4, R-S, and R-6, the T pad being connected across resistor R-16 and thus to the transformer 16 when the switch 26 is in its upper posi-tion.

Magnetic sound channel No. 3 is connected through conductors 29 to speaker 30 positioned at the right-hand side of the motion picture screen. Bridged across the conductors 29 is T pad 32 composed of resistors R-7, R-S, and R-9. Source No. 3 may be monitored when pad 32 is connected to transformer 16 by switch 33, as shown in the drawing.

Photographic sound track channel No. 4 is connected through conductors 35 to side speakers 36, 37, and 38. Bridged across conductors 35 is a T pad 40 composed of resistors R-lfl, R-11 and R-12. This photographic channel No. 4 may be monitored by connecting T pad 40 to the amplifier 15 when the switch 41 is in its upper position.

The No. 5 photographic sound track channel is an emergency channel and is connectable by conductors 43 to the center speaker 23 when the switch 22 is in its lower posi-tion. That is, in the advent of a break-down in any stereophonic sound channel, switch 22 will be thrown -to its lower position and a composite track in reproducer No. 5 will be impressed on the center speaker 23. For monitoring reproducer No. 5, conductors 43 are bridged by a T pad 45 composed of resistors R-13, R-14, and R-15. The pad is connected to the amplifier 15 when the switch 46 is in its upper position.

As mentioned above, the above monitoring system prevents cross-talk between various channels and maintains the sound level constant in all speakers, both theater and monitor. Formerly, monitoring was accomplished by the use of a low gain bridging amplifier which was switched to the various channels. This system is unsuitable for multiple channel systems, since it cannot be used to check two or more channels at once, as it would effectively short-circuit these channels and ruin the stereophonic effect.

With the present system, which uses the high gain amplifier 15, high impedance bridging pads 12, 25, 32, 40, and 45 may be used between the high gain amplifier and the outputs of the respective sound channels. This makes loading of the monitor circuit on the outputs of the main sound channels negligible, which gives no reduction in level in the main theater speakers regardless of the number of channels being monitored.

Cross-talk between the channels is also negligible due to the double isolation between channels by the series connection of the T pads above mentioned when the switches are thrown for monitoring two or more channels at once, such as shown at 26 and 33. The matchingk resistor R-16 is used on the input of the monitor amplifier, not only to load the input of the amplifier properly for a fiat frequency response curve, but also, to serve as a terminating resistor for the T pads. That is, a single resistor serves to Iterminate one or as many channels as are switched into the monitoring circuit. This provides a reverse bridging effect on the monitor amplifier resulting in approximately the same sound level in the monitor speaker regardless of the number of channels connected for monitoring. Although single switches 13, 26, 33, 41, and 46 have been shown, it is obvious that a single rotary switch could be substituted. These switches may be usedV for monitoring the various channels as may be desired.

d In one system which provided satisfactory results, the value of resistors R41, R-Z, Red, R-S, R-71, R-S, R40, R-ll, R-lS, and R-14 was 40,000 ohms each, and resistors R3, R6, -`RJ, R-IZ, R-IS, and R-16. were 500 ohms each. The monitor amplifier 15 provided 100 db gain.

I claim:

l. A combination of a plurality of independent sound channels, a monitoringcircuit including a high gain amplitier, a resistor in the input of said amplifier, and a loudspeaker, and interconnecting means between said resistor and said independent sound channels for interconnecting said channels in different combinations to said monitoring circuit, sa-idl resistor having a value to load the input of said amplifier to provide a tlat frequency response and a terminating resistor for said interconnecting means to provide a substantially constant level in said loud speaker regardless of the number of said sound channels connected to said ampliiier.

2. A combination in accordance with claim l, in which said last mentioned means includes a: high impedance bridging T pad across the output 'of each of said channels, saidl pads being connectable in series to said ampliier, the number of said pads connected in series depending upon the number of channels being simultaneously monitored.

3. A monitoring system for a plurality of separate sound channels for monitoring one or more of said channels simultaneously comprising a high impedance monitoring amplifier, amatching resistor across the input of said amplifier, a plurality of separate sound channels, and means for interconnecting said channels across said matching resistor individually and in groups at will for monitoring one chan-nel and groups of channels simultaneously, said matching resist-or having a value to load the input of said amplifier to provide a flat frequency response and a terminating resistor for said interconnecting means to provide a substantially constant level in said loud speaker regardless ofV the number of said sound` prising a plurality of audio sound reproducing channels having separate speakers, a high impedance T pad bridged across each of said channels, a high impedance ampiiier, a speaker connected to the output of said ampliiier, and switch means for connecting any number of said channels to said amplifier at any one time, said T pads being connectable in series by said switch means to maintain the level in said single monitor speaker substantially constant regardless of the number of reproducing channels connected to said high impedance amplifier. 6. A system in accordance with claim 5 in which a matching resistor' is provided across the input of said ampliiier, said channels being connectable across said resistor, saidy resistor serving as a terminating resistor for said T pads and a load for the input of said amplifier for providing a flat frequency response for said amplifier and for maintaining a substantially constant level in said single monitor speaker regardless of the number of said reproducing channels connected to said amplier.

References Cited in the iile of this patent UNITED STATES PATENTS 1,540,317 Craft et al. June 2, 1925 1,799,235 Humphrey Apr. 7, 1931 2,315,784 Goodale Apr. 6, 1943 2,499,603 Mueller Mar. 7, 1950 'een n 

